Rollup merge of #56016 - scottmcm:vecdeque-resize-with, r=joshtriplett

Add VecDeque::resize_with

This already exists on `Vec`; I'm just adding it to `VecDeque`.

I wanted to resize a `VecDeque<Vec<T>>` when I didn't know `T: Clone`, so I couldn't use `.resize(n, Vec::new())`.  With this I could do `.resize_with(n, Vec::new)` instead, which doesn't need `T: Clone`.

Tracking issue: https://github.com/rust-lang/rust/issues/41758

src/liballoc/collections/vec_deque.rs

@@ -19,7 +19,7 @@
 
 use core::cmp::Ordering;
 use core::fmt;
-use core::iter::{repeat, FromIterator, FusedIterator};
+use core::iter::{repeat, repeat_with, FromIterator, FusedIterator};
 use core::mem;
 use core::ops::Bound::{Excluded, Included, Unbounded};
 use core::ops::{Index, IndexMut, RangeBounds};
@@ -1920,6 +1920,44 @@ impl<T: Clone> VecDeque<T> {
             self.truncate(new_len);
         }
     }
+
+    /// Modifies the `VecDeque` in-place so that `len()` is equal to `new_len`,
+    /// either by removing excess elements from the back or by appending
+    /// elements generated by calling `generator` to the back.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_resize_with)]
+    ///
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(5);
+    /// buf.push_back(10);
+    /// buf.push_back(15);
+    /// assert_eq!(buf, [5, 10, 15]);
+    ///
+    /// buf.resize_with(5, Default::default);
+    /// assert_eq!(buf, [5, 10, 15, 0, 0]);
+    ///
+    /// buf.resize_with(2, || unreachable!());
+    /// assert_eq!(buf, [5, 10]);
+    ///
+    /// let mut state = 100;
+    /// buf.resize_with(5, || { state += 1; state });
+    /// assert_eq!(buf, [5, 10, 101, 102, 103]);
+    /// ```
+    #[unstable(feature = "vec_resize_with", issue = "41758")]
+    pub fn resize_with(&mut self, new_len: usize, generator: impl FnMut()->T) {
+        let len = self.len();
+
+        if new_len > len {
+            self.extend(repeat_with(generator).take(new_len - len))
+        } else {
+            self.truncate(new_len);
+        }
+    }
 }
 
 /// Returns the index in the underlying buffer for a given logical element index.